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Section 1.3 Ordered Pairs and Functions

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Section 1.3 Ordered Pairs and Functions Section 1.3 Ordered pairs and functions The ordered pair (a, b) of two things a and b is another thing that contains the information of both a and b , together the information that " a comes first, b second". Mathematically expressed, the essential property of the ordered-pair construction is (a, b) = (c, d) ¢¡£¡£¤ a = c and b = d . (1) It is possible to construct the ordered pair set-theoretically; however, we will not do so here; all we ever use about ordered pairs is the fact expressed in (1). Let us note though that the pair-set {a, b} would not work as the ordered pair: we have {0, 1} = {1, 0} , but we want (0, 1) ≠ (1, 0) . The use of the ordered pair is familiar in coordinate geometry; the points in the plane equipped with a Cartesian coordinate system are represented by ordered pairs of real numbers. Various geometric figures become sets of ordered pairs. Denoting the set of ordered pairs of real 2 numbers by ¥ , the set 2 2 2 {(x, y)∈ ¥ : x + y = 1} is the circle with center the origin, and radius the unit length; that is, the set of points on that circle. The set 2 2 2 {(x, y)∈ ¥ : x + y < 1} is the open disc of radius 1 around the origin; 15 2 2 2 ≤ {(x, y)∈ ¥ x + y 1} is the closed disc; the open disc does not, the closed one does, contain the circumference. For sets A and B , A × B , the Cartesian product of A and B , is the set of all ordered pairs (a, b) with first element a from A , second element b from B : A × B = {(a, b) : a ∈ A and b ∈ B} . def 2 2 ¥ × ¥ Thus, what we wrote as ¥ above is the same as ; in general, we may write A for A × A . A function f from a set A to another set B is a rule that assigns, to every element a of A , a definite element of B ; this element is denoted by f(a) ; it is called the value of the function f at the argument a . We write ¢¤¢¤¢ ¢¤¢¤¥ f:A ¡£¢ B to indicate that f is a function from A to B ; A is the domain of f , B is the codomain of f . The codomain of f has to be distinguished from the range of f ; the latter is the set {f(a):a∈A} of all values of f : range(f) = {f(a) : a ∈ A} . def The range of f is a subset of the codomain of f ; the range and the codomain may or may not be the same. It is possible to construe functions as sets, in particular, as sets of ordered pairs: with ¢¤¢¤¢ ¢¤¢¤¥ ∈ f:A ¡£¢ B , we may consider the set of all pairs (a, f(a)) with a A ; this set is called the graph of the function f : graph(f) = {(a, f(a)) : a ∈ A} . def This is exactly the representation of functions that we use in coordinate geometry and calculus. 16 x ¡£¢ ¥ ¥ For instance, with the exponential function exp: ¥ assigning e to x for all x ∈ ¥ , we associate its graph which is the exponential curve in the Cartesian plane. ¡£¢ ¥ ¥ Note that the range of exp: ¥ is the set of all positive real numbers, + ∈ ¥ ¥ = {y : y>0} . This is true since the values of the exponential function are all positive, and every positive real number is the value of exp at a suitable argument x∈ ¥ : if y>0 , ¥ ¡£¢ ¥ ¥ then there is x∈ ¥ namely, x=ln(y) , for which y=f(x) . The range of exp: + ⊂ ¥ does not coincide with its codomain: ¥ ≠ . Usually, we do not distinguish between the function and its graph; the exponential function and the exponential curve are considered to be the same thing. There is one qualification to ¢¤¢¤¥ ¡£¢ ¢¤¢¤¥ this rule though: two functions f:A ¡£¢ B and g:A C , with the same domain but with different codomains, may have the same graph. E.g., the sin function may be construed as ¡£¢ ¢¤¢¤¥ ¥ ¥ ¥ ¥ ¡£¢ ¢¤¢¤¥ ¥ sin: ¥ , from to , or as sin: [-1,1] , from to the closed interval [-1, 1] (since all values of sin are in the latter interval); these two functions have the same graph. For us, these two functions are technically different; the specification of a function includes the specification of its domain as well as its codomain. When is a set, say A , is the graph of a function? There are two conditions that are necessary and sufficient for this to hold: (i) Every element a of A must be an ordered pair: a must equal to (x, y) for suitable (uniquely determined) x and y ; (ii) For all x, y, z , (x, y)∈A and (x, z)∈A imply that y=z [note the same x as first component in the two ordered pairs]. The second condition expresses the fact that for a function f , the value y=f(x) is uniquely ¥ determined by the argument x . If (i) and (ii) hold true, then there is a function f:X ¡£¢ Y for which graph(f)=A . Here, X , the domain of f , is the set of all x for which there is y such that (x, y)∈A ; Y , the codomain, is any set that contains as a subset the set R of all y for which there is x such that (x, y)∈A ( R is the range of f); and we have y = f(x) ¢¡£¡£¤ (x, y) ∈ A . x The usual notation for a function is to give its value at an indeterminate argument; thus, e 17 denotes the exponential function. This notation is ambiguous, however; it may also mean the value of the function at a certain argument-value of x . A more explicit notation e.g. for the exponential function is x ¢ ¢ ¢ ¥ ∈ ¥ x ¢ e (x ) Note here the vertical line at the beginning of the arrow; this kind of arrow is to be distinguished from the arrow that connects the domain and codomain of the function. If we write exp for the exponential function, a full notation and description of the function exp is this: ¡£¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ ¥ exp : ¥ x ¢ ¢ ¢ ¢ ¥ x ¢ e . ¢¤¢¤¥ ¡£¢ ¢¤¢¤¥ If we have two functions f:A ¡£¢ B and g:A B between the same two sets, f and g are the same function, f = g , just in case for all a ∈ A , f(a) = g(a) : f = g ¢¡£¤ for all a ∈ A , f(a) = g(a) . This is in agreement with the construal of functions as sets of ordered pairs: f = g just in case graph(f) = graph(g) ; note that this is valid only if the two functions f and g are given already with the same domain and the same codomain. Here is a notation for specifying a function when the domain of the function is a reasonably small finite set. I©ll explain this on an example. For instance, the symbolic expression 1 3 5 7 9 11 ( ) (2) 0 5 4 20 3 3 denotes the function whose domain is the set {1, 3, 5, 7, 9, 11} , the set which is listed in the upper row, and whose value for each argument in the domain is given in the second row underneath the particular argument; in the case of (2), if the function is called f , then f(1)=0 , f(3)=5 , f(5)=4 , etc. To be precise, we should note that this notation exhibits only the graph of the function. In the 18 example (2), the function f may have any codomain (which then has to be specified separately) that contains the set {0, 5, 4, 20, 3} , the range of the function f . ¢¤¢¤¥ ¡£¢ ¢¤¢¤¥ If we have two functions, f:A ¡£¢ B and g:B C , such that the codomain of the first is ¡£¢ ¢¤¢¤¢ ¢¤¢¤¥ the same as the domain of the second, we can form their composite g f:A C ; the definition of g f is: ∈ (g f)(a) = g(f(a)) (a A) . def We may omit the circle in the notation of composition, and write simply gf . To see the domain/codomain relationships of the functions involved, we may draw the three functions f , g , and gf in the diagram ¢ £¥¤ ¡ B ¤ ¨ f¡§¦ g ¤ ¡§¦ ¨ © ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ A ¡£¢ C gf The composite of two functions is defined only if the codomain of one coincides with the domain of the other. E.g., consider the functions ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ ¡£¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ f : ¡£¢ ( ) and g : ( ) ( ) ¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ ¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ n {n} X - X Then, gf is the following function: ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ gf : ¡£¢ ( ) ¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ ∈ ≠ n {x x n} x When, in the calculus, we talk about a function like sin(e ) , we have in mind a composite; in the case at hand, the composite sin exp : 19 exp sin ¥ ¡£¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ ¥ ¡£¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ ¥ x ¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ x e ¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ y sin(y) x x ¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ ¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ x e sin(e ) The operation of composition of functions satisfies the associative law: in the situation f g h ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ ¡£¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ ¡£¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ A ¡£¢ B C D , we have that h(gf) = (hg)f . Indeed, h(gf) applied to any a ∈ A gives [h(gf)](a) = h([gf](a)) = h(g(f(a))) ; and, (hg)f applied to a gives [(hg)f](a) = (hg)(f(a)) = h(g(f(a))) , which is the same value. Since the two functions, both from A to D , give the same value at each argument a ∈ A , they are equal. With any set A , there is a particular function associated, namely the identity function on A : ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ 1A : A ¡£¢ A ¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ a a ∈ ( 1A(a) = a for a A ). This has the property that its composite with any function, provided it is well-defined, is the function itself: 1 f A ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ ¡£¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ B ¡£¢ A A :: 1A f = f , 20 1A g ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ ¡£¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¢ ¢¤¢¤¥ A ¡£¢ A C :: g 1A = g .
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